How can I read a file in node js, find all instances of a function and then extract each function's argument?

Question

I'm trying to write a node script that identifies unused translation strings in my React project.

First, I want to get a list of all the translations that are used. To do this, I am getting a list of each JS file in my /src/components folder and then reading the file.

My translation strings look like this: t('some.translation.key'), so basically, I want to identify each instance of t('...') using RegEx and then get the key in between those parentheses (i.e. "some.translation.key"). From there, I should be able to compare the keys to the ones in my translation JSON file and remove the ones that aren't being used.

unused.js

const path = require('path'); const fs = require('fs'); let files = []; // https://stackoverflow.com/a/63111390/2262604 function getFiles(dir) { fs.readdirSync(dir).forEach(file => { const absolute = path.join(dir, file); if (fs.statSync(absolute).isDirectory()) { getFiles(absolute); } else { if (absolute.includes('.js')) { files.push(absolute); } } }); return files; } function getTranslations() { const pathComponents = path.join(__dirname, '../../src/components'); // get all js files in components directory const files = getFiles(pathComponents); const translationKeys = []; // for each js file for(let i = 0; i < files.length; i++) { // read contents of file const contents = fs.readFileSync(files[i]).toString(); // search contents for all instances of t('...') // and get the key between the parentheses } } getTranslations();

How can I use RegEx to find all instances of t('...') in contents and then extract the ... string between the parentheses?

CherryDT · Accepted Answer · 2022-04-11 22:20:26Z

Yes, you could use a regular expression:

for (const [, str] of contents.matchAll(/\bt\(['"](.*?)['"]\)/g)) { console.log('t called with string argument:', str) }

However, with regular expressions the problem will be that they don't understand the code and would cause trouble with matching strings that contain ( ) or \' themselves, have issues with concatenated strings or extra whitespace, etc., and you'd then also get the contents literally, including possible escape sequences.

A more robust way would be to create an AST (abstract syntax tree) from the code and look for calls to t in it.

A popular AST parser would be acorn. There is also the supplementary module acorn-walk that helps walking through the whole syntax tree without building your own recursive algorithm.

import acorn from 'acorn' import walk from 'acorn-walk' // Example const contents = "function a () { if (123) { t('hello') } return t('world') }" // The arguments to acorn.parse would have to be adjusted based // on what kind of syntax your files can use. const result = acorn.parse(contents, {ecmaVersion: 2020}) walk.full(result, node => { if (node.type === 'CallExpression' && node.callee.type === 'Identifier' && node.callee.name === 't') { if (node.arguments.length === 1 && node.arguments[0].type === 'Literal' && typeof node.arguments[0].value === 'string') { // This is for the case `t` is called with a single string // literal as argument. console.log('t called with string argument:', node.arguments[0].value) } else { // In case you have things like template literals as well, // or multiple arguments, you'd need to handle them here too. console.log('t called with unknown arguments:', node.arguments) } } }) // Will output: // t called with string argument: hello // t called with string argument: world

Thank you for the thorough answer! This helps a lot. In my case, I'm using React and JSX, so I needed the acorn-jsx and acorn-jsx-walk packages. I also needed sourceType: 'module' as a parser option to use require(). With those in place, I'm off to the races!
Exactly what I'm looking for (finding translations with t()), but acorn doesn't seem to support TSX. Any idea what I could do?

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